The Orthogonal Frequency Division Multiple Access (OFDMA) systems are similar to Orthogonal Frequency Division Multiplex (OFDM) systems in the sense that the information is spread over multiple subcarriers in the frequency domain and is transmitted after converting the information to the time domain using a Fourier transform. The amount of subcarriers available to a system is determined by a ratio of the assigned frequency channel to the frequency spacing between the subcarriers.
The main difference between OFDM and OFDMA is that with OFDM a single transmitter uses a whole range of subcarriers to transmit its information, while in OFDMA different transmitters are assigned disjoint sets of subcarriers, and each user sends his information on the subcarriers assigned to him.
The amount of subcarriers assigned to each transmitter may vary according to the traffic demands of each user in a multiple access system. The assignment is usually performed in groups of subcarriers, denoted as subchannels. The OFDMA idea became popular recently in the context of wireless access systems. There are several variants of division of the subcarriers into subchannels, starting from irregular methods such as in IEEE802.16a [4] and in DVB-RCT [5], through contiguous clusters of subcarriers [3] and concluding with regularly interleaved sets.
In burst communication systems each transmission typically starts with a preamble, which is used for synchronization and channel estimation. Usually such preamble is structured as one or two OFDM symbols with predefined values modulating each of the subcarriers. The set of predefined values is chosen to satisfy several criteria. One is that all the values have the same modulus, to ease the channel estimation. Only the values corresponding to the assigned subcarriers are non-zero. Another criterion is that the preamble time domain waveform has a low Peak-to-Average Power Ratio (PAPR) property, to avoid excessive distortion in the power amplifier.
Typically OFDM systems produce high PAPR waveforms, since at each time instant numerous data-dependent contributions add up to a Gaussian-like waveform. As a result, the OFDM transmitters utilize their power amplifiers at a small fraction of their peak output, typically at 8-11 dB backoff. By using as a preamble, a carefully crafted set of subcarrier modulation values, the PAPR of the preamble can be kept at about 3 dB, significantly less than the values typical to data. This property is beneficial in that during the channel estimation phase the signal experiences smaller distortion, resulting in a more accurate estimate.
The preambles in OFDMA systems are designed to excite only the subcarriers that are assigned to the user. The OFDMA adds a new twist to the problem, since not only a single preamble with good PAPR properties needs to be designed, but rather a family of preambles for each subset of subcarriers that can be allocated to a single transmitter.